In VHDL, a single apostrophe ' is used in the following case: <signal name>'event where <signal name> can be any predifined signal (its like a variable). The bug is of course that the VHD highlighter thinks this is a single quote opeing a string, thus highlighting the rest of the line.

An easy rule would be to just check for the word "event" after a single quote.
Here is the required update for vhd.xml - the following should replace the single quote span:
<SPAN_REGEXP HASH_CHAR="'" NO_LINE_BREAK="TRUE" TYPE="LITERAL1">
<BEGIN>(?!'event)</BEGIN>
<END>'</END>
</SPAN_REGEXP>